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Battery apps benefit from MCU clocking, mode control

Posted: 16 Oct 2003 ?? ?Print Version ?Bookmark and Share

Keywords:mcu? microcontroller? clocking? batter? power?

We have come to expect and require more from our battery-powered devices. In tandem with this increase in system sophistication, the battery-power requirements are increasing while battery chemistries are not keeping up. This increase of functionality is only accomplished if a savvy firmware/software engineer understands the tools available in the MCU as well as hardware options. As a result, firmware designers are forced to use what is on hand today to optimize power consumption. This can be done by using power-efficient hardware combined with MCU mode controls.

A simple battery-operated system with an MCU provides a useful example. The central focus for a successful low-power design is an MCU of this type. The attributes of the MCU by itself include idle and sleep modes where system power can be conserved. The idle modes of the MCU power down the CPU while allowing peripheral functions such as the 10bit ADC to continue to operate. The sleep mode implements a complete shutdown of the controller.

The MCU's clock power dissipation, and more importantly, startup time will determine what type of clock is selected for the application. The choices include an R/C loop, a crystal oscillator or a resonator. For example, the startup time of a 32kHz crystal oscillator is 400ms to 900ms. During this startup time, the MCU is drawing power from the battery. In contrast, the typical startup time of internal R/C oscillators is in microseconds. With the internal R/C oscillator, code can be executed immediately after the MCU comes out of its sleep mode, but the R/C oscillator is not as accurate as an external crystal if critical timing events are required.

The clock source's power dissipation during startup can be reduced by using a two-clock strategy with the MCU. An MCU two-clock function is implemented by using the clock with a faster startup time, in this case the internal R/C oscillator, to execute code while the more accurate clock is starting up in the background. If it is determined by the MCU that the second more accurate clock is not required, it is turned off. On the other hand, if the second, higher accuracy clock is required to execute critical time events, it is allowed to complete its startup time cycle and the first clock is turned off.

MCU manufacturers are adding modes that save average power over long periods of time. The combination of lower power peripherals and MCU modes enhance the chances of having a low battery-powered solution.

Optimizing these independent peripheral power trade-offs device by device is important, but the real power savings can be found when the external and internal peripherals are used in concert with the MCU mode programming capability. For instance, the MCU can be used to control the power supply voltage by switching a new configuration into the resistive feedback system of the regulated adjustable output charge pump.

A higher output voltage may be needed from the charge pump to ensure that the analog circuitry performs at its optimum level. A lower voltage may be desirable when only digital events are occurring.

Individual power savings from each device in a battery-powered application is extremely important if battery life is a critical issue. But, true value is achieved when the MCU's programmable capability is exploited. A few areas where this could be done would be to change the power supply voltage at the output of a regulated charge pump, power down non-critical peripherals when not in use and controlling the clocking strategy to optimize power vs. functionality. IC manufacturers are continuing to improve the dynamic performance of their peripheral devices while reducing the quiescent current and supply voltage requirements.

Lower power is achieved by reducing power supply requirements and quiescent currents, by optimizing topologies for the lower power jobs. In the example shown here, an operational amplifier, and an ADC are the managed peripherals, while power applied by a regulated, adjustable charge pump is examined.

Power push

The operational amplifiers are designed using CMOS processes. These types of op amps are continuing to push minimum power supply voltage requirements down. For handheld data acquisition, a 14kHz, 600nA amplifier can function on single-supply voltages as low as 1.4V and up to 5.5V. The combination of reduced supply voltage and lower quiescent provides a good solution for power management concerns in battery-operated equipment.

With internal or external integrated ADCs, the amount of power dissipated is more dependent on the converter topology than on IC design innovation. For instance, the ratio of conversion time to current consumption of the Successive Approximation Register (SAR) converter is considerably lower than a common alternative, the delta-sigma converter. In battery-powered applications, the SAR converter is usually used unless high resolution and accuracy is an absolute necessity.

The power supply to the circuit may need to be adjustable. A higher voltage of 5V is best suited for analog circuitry and a lower voltage of 2V is acceptable for digital activities. If an adjustable converter is used, it should be optimized for high efficiency with low-output currents and Li-ion battery input voltages (4.2V down to 2.8V). The regulated, adjustable charge pump, DC/DC converter (such as the MCP1252-ADJ in this example) is selected for this application for these reasons, but alternatives can be used dependant of the particulars of the application's battery output voltage and power requirements.

- Bonnie C. Baker

Applications Manager

Microchip Technology Inc.

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